[AMC-list] Oil Pressure

["Bruce Hevner" <scramblr@xxxxxxxxxxx>]

Oil pressure depends on several factors.

 

Bearing clearances. Was the engine built for high performance with increased
bearing clearance, or street use with tight bearings.

 

Oil pump. How tight were the clearances and was the pressure relief spring
tension increased (only affects high end pressure NOT idle pressure)

 

Type of oil used. Weight and type of oil. Heavier weight standard oil will
have higher pressure.

 

MOST engines with street clearances will have at least 10-20 lbs at hot
idle. Back when the GM3800 was known as the 3.8 they had the oil pump in the
front cover a lot like the AMC. Idle oil pressure was a common problem with
them and it wouldn't be uncommon to see them with the lifters bleeding down
and LESS than 3lbs OP at idle!!! There was a high capacity OP kit for them
which could be installed in the car which increased the OP capacity by about
20% and usually cured the problem. GM's answer was that unless the lifters
were noisy, or the OP light was on, to install a new OP switch which didn't
turn the light on until LESS than 3lbs !!!! (stock one was 5) It sounds bad
but I saw many, many of these motors go 100,000 miles like that. When they
went to the 3800 they completely redesigned the OP (driven by the crank NOT
the cam) and cured the problem.

 

So engines CAN live with relatively low OP at idle as long as it comes up to
spec with RPM. MOST stock OPs will deliver "about" 10-15lbs per 1000 RPM and
top pressure will be "about" 40-50 with a stock relief spring. If it's a HP
motor with increased bearing clearances the OP needs to be modified to keep
up with the increased oil flow. Remember oil flow past the bearings is NOT
linear. It increases at "about" 3 times the increase in clearance. A .001
increase in clearance will require THREE times more flow !!!!!! 

 

I like to see my modified motors with at least 20 at idle and then hit the
relief within a few thousand RPM. Too much pressure can be a problem. It
puts a larger load on the drive gear which can result in cam gear failure
and "wash out" bearings.  Small block Fords (with their narrow rod bearings)
need more than Chevys so it also depends on the application.

But HEY,,,, that's just ME!!!

Bruce Hevner 

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